Abstract. Mixing ratios of water (H2O) in the stratosphere appear to increase due
to increased input of H2O and methane from the troposphere
and due to intensified oxidation of CH4 in the stratosphere, but many of the
underlying mechanisms are not yet understood. Here we identify and quantify
three chemical mechanisms which must have led to more efficient oxidation of
CH4 in the stratosphere over the past several decades: 1) The increase in
stratospheric chlorine levels due to anthropogenic CFC emissions, 2) the thinning of the
stratospheric ozone column and 3) enhanced OH levels in the stratosphere due
to increasing H2O levels themselves. In combination with the increase
in tropospheric CH4 mixing ratios and with solar cycle related
variations of upper stratospheric ozone, these effects can explain about
50% of the additional conversion of CH4 to H2O as observed
throughout the stratosphere. The relative contributions from the individual
processes have varied over the past decades.